In order to make cast-in-situ piles in the ground, it is known that holes must initially be bored presenting the diameter and the depth that correspond to the pile that is to be made, after which concrete or a grout is injected into the borehole so as to make the pile.
French patent No. 2 566 813 describes an improved method of boring holes for cast-in-situ piles in which a machine is used as shown in accompanying FIG. 1. The machine comprises an assembly which is suspended from an end of a cable supported by the jib of crane. The machine has a top head 1. The top head 1 forms a bracket using a system of actuators 2 and 3 for supporting a drive head 4 for setting an auger 5 into rotation. The auger is constituted by a hollow core 6 and by a helically-shaped blade 7. The drive head 4 serves to turn the auger 5 thus enabling a hole 7′ to be bored. A rigid tube referred to as a “dip” tube 9 is slidably mounted inside the hollow core 6 and the bottom end of the tube is provided with a tool 14 and with a hole 13 for injecting concrete. The top end of the tube is secured to the support bracket 1 and is connected to a hose 10 for delivering concrete via a rotary joint 15.
While boring, the dip tube is constrained to move in translation and in rotation with the auger 5, e.g. by a clutch system 12. Under such circumstances, the actuators 3 are extended so that the tool 14 is at the bottom end of the core 6 of the auger. When boring is terminated, the dip tube is separated from the auger.
In order to make the cast-in-situ pile in the hole 7′, the actuators 3 are operated to raise the auger progressively relative to the dip tube 9 which is then free to turn. Thus, concrete can be injected into the borehole via the orifices 13 formed at the bottom end of the dip tube since these orifices are now outside the auger. Starting from this position in which the dip tube is offset relative to the auger, the entire dip tube and auger assembly can be raised using the crane so as to allow the borehole to be filled completely with concrete or grout.
That technique of making cast-in-situ piles suffers from the drawback of not enabling the quality of the boring to be controlled effectively, in particular the quality of the walls of the borehole and specifically the quality of the bottom end of the borehole. Unfortunately, the quality and the mechanical strength of the pile depend specifically on the excavated material that comes from making the borehole not being mixed with the concrete or grout while it is being injected into the borehole.